Trim and discard fat from tri-tip. Rinse and pat dry, then sprinkle with garlic salt and pepper.
Lay meat on a hot oiled grill (you can hold your hand at grill level only 1 or 2 seconds). Cook, turning as needed to brown evenly, until a thermometer inserted in the center of the thickest part reads 125 to 130, about 20 minutes.
Transfer the meat to a cutting board and let rest 15 minutes.
Cut meat across the grain into thin, slanting slices.